Meiosis is the two-stage form of cell division that produces four haploid cells from a single diploid cell (but see the note on oogenesis below). In the process, it divides a single nucleus, containing two sets of replicated chromosomes (chromosomes composed of two sister chromatids), into four nuclei, each containing a single set of unreplicated chromosomes. These steps are the phases, or stages of meiosis.
Overview of meiosis:
During meiosis one haploid cell divides two times to produce four haploid daughter cells. Meiosis begins with prophase I, during which the chromosomes, which were up to that point thin, unpacked threads invisible under a light microscope, begin to pack (condense) into dense visible masses. Homologue pairs then attach ("synapse") to form "tetrads" and exchange DNA ("cross-over"). Next, during metaphase I, the cellular machinery attaches itself to the tetrads and moves them to the equator of the cell ("metaphase plate"). In anaphase I, the two chromosomes from each tetrad separate and move to opposite ends ("poles") of the cell. During telophase I, they reach the poles and the cell separates into two daughter cells. The preceding process, known as meiosis I, is followed by meiosis II, during which the two sister chromatids of each chromosome separate (as in mitosis) and move to opposite poles. Also during telophase the cell elongates and ultimately separates into two daughter cells. Meiosis is then complete.
When does meiosis occur?
Meiosis occurs only in eukaryotes. In animals, it occurs during the production of gametes. In plants, it takes place when spores are produced (plant gametes are produced by mitosis). Meiosis does not occur in prokaryotes (i.e., archaea and bacteria). They reproduce by binary fission.
Why is meiosis necessary?
Meiosis has to take place at some point in a sexual reproductive cycle because it halves the chromosome number, which compensates for the doubling of the chromosome number caused by fertilization.
In the case of oogenesis, a single egg cell and three polar bodies are produced by meiosis.
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